Genetics II
Non-Mendelian Inheritance
Non-mendelian
Genetics
Mendelian
Genetics
•
•
•
•
•
•
•
• Complete
dominance
Codominance
Incomplete dominance
Multiple alleles
Pleiotropy
Epistasis
Polygenic inheritance
Sex-related inheritance
Pleiotropy
(pleion, “more”)
Ability of a gene to affect an organism in multiple ways
Examples:
– 40% of cats with white fur and blue eyes are deaf
– Phenylketonuria in humans (PKU)
• Lack of an enzyme that converts Phe  Tyr
• Mental retardation, reduced hair and skin
pigmentation, eczema
– Sickle-cell anemia
• Problem with hemoglobin molecules  misshapen
RBC’s at low O2 concs  kidney and bone damage
Epistasis
One gene affects expression of another
• Effects of one gene are
modified by one or several
other genes called modifier
genes
• Gene whose phenotype is
expressed  epistatic
• Examples:
– Coat color in mammals
– Color of wheat grains
– Fruit color in squash
– Bombay phenotype in
ABO blood groups in man
Phenotypes
A
O
O
Genotypes:
AO
OO
OO
Bombay Phenotype
A
AB
O
Bombay Genotype
AA (Hh)
AB (Hh)
AB or AA (hh)
Polygenic Inheritance
Additive effects of two or more genes on a phenotypic character
• Quantitative characters
vary in a population
along a continuum or
gradation
• Due to the presence of
contributory (ABC) and
non-contributory alleles
(abc)
• Expression can be
affected by
environmental factors
• Example:
– Skin pigmentation
controlled by at least
three separately
inherited genes
– Each dark skin allele
contributes a unit of
‘darkness’ to the
individual
– AABBCC  very dark
– aabbcc  very light
Polygenic Inheritance Problem Solving
Petal length of a plant ranges from
4mm to 12mm to 20mm. Out of
770 plants, only 3 of them have
4mm petals.
1. Give one genotype for a plant
with 12 mm petals.
2. Give two possible genotypes for
plants with 6 mm petals
3. What proportion of plants have
14 mm petals?
4. What is the phenotype of plants
with 7 contributory alleles?
5. How many contributing alleles
does a plant with 4mm petals
have?
Sex-related
Inheritance
expression of traits is affected by the sex
of the individual
•
Sex is an inherited
phenotypic character
determined by the
presence/absence of certain
chromosomes
• SRY (sex-determining region
of Y) in humans
1. Sex-linked inheritance
2. Sex-limited inheritance
3. Sex-influenced inheritance
44 +
XY
44 +
XX
Parents
22 + Sperm 22 +
Y
X
22 +
XY
Ova
44 + Zygotes
44 +
XX (offspring) XY
(a) The X-Y system
22 +
XX
22 +
X
76 +
ZW
76 +
ZZ
(b) The X–0 system
(c) The Z–W system
32
(Diploid)
(d) The haplo-diploid system
16
(Haploid)
Sex-linked
Inheritance
• Gene linkage – genes
located on the same
chromosome are
inherited together
• Sex-linkage
– Sex chromosomes
contain genes for
many characters
unrelated to sex
– X-linked/Y-linked
gene
(1) Father
affected with
the disorder Xa,
Mother is
homozygous
dominant
(2) Carrier
mates with a
male of normal
phenotype
(3) Carrier
mates with a
male with the
disorder
Examples of X-linked alleles
Recessive alleles
• Color-blindness
• Duchenne muscular
dystrophy
• Hemophilia
• Testicular feminization
Dominant traits
• Hypophosphatemia
Mammalian females
• One of the two X chromosomes is randomly inactivated
• If female is heterozygous for a gene located on the X
chromosome, she is a mosaic
Two cell populations
in adult cat:
Active X
Early embryo:
X chromosomes
Cell division
and X Inactive X
chromosome Inactive X
inactivation
Allele for
black fur
Active X
Orange
fur
Black
fur
Examples of Y-linked (Holandric)
Inheritance
• Transmission of genes from father to son
– Testis-determining factor (TDF/SRY gene)
– hypertrichosis
Sex-limited Inheritance
• involves autosomal genes that are expressed
only in either males or females
• resulting in a part or function of the body that is
present in one sex but not the other
• e.g. milk production
cryptorchidism
feathers in domestic fowl
Genotype
HH
Hh
Hh
Female
hen-feathered
hen-feathered
hen-feathered
Male
hen-feathered
hen-feathered
rooster-feathered
Sex-influenced Inheritance
• dominant in one sex but
recessive in the other
• autosomal
• difference in expression due
to the hormonal difference
between the sexes
– in heterozygotes, the
expression of the trait is
affected by sex hormones
– homozygotes unaffected
and express the trait
regardless of the
hormone produced
e.g. pattern baldness*
gene for hair growth pattern has 2
alleles: one that produces hair all over
the head and another that causes
pattern baldness
- the baldness allele is dominant in
males but recessive in females
- a heterozygous male is bald, but a
heterozygous female is not
Genotype Male (dominant)
Female (recessive)
b1b1
bald
bald
b1b2
bald
not bald
b2b2
not bald
not bald